Dark Energy
Some think Einstein's cosmological constant is evanescent subatomic pariticles poping in and out of existence causing "strechiness" that pushes "empty space" apart. Some think that this roiling sea of particles is the best candidate for dark energy. The problem is that the calculated energy of the roiling sea is 100,000 + 51 zeros times more than than the dark energy. So to account for things being as they are, something has to cancel out all this energy from the poping particles down to a tiny tiny tiny smidgen. Nobody knows what does this and what is more, the precision of the required cancellation--like erase the ink on every book ever printed except for the period at the end of this sentence strains all credulity. This spawned the multiuniverse theory in which this feature allows for us and others do not. However, this leaves a "just is" explaination of things, so others are purposing more exotic solutions like "leaking gravity." Where does it go?--hidden dimensions are the best candidate so far.